Optical device determining the position and orientation of an object, and optical recording and/or reproducing apparatus including the device

ABSTRACT

An optical system for determining the position and/or orientation of an object (10), for example, a freely suspended rotatable polygon mirror, comprises a convex mirror (24) on the rotation axis (11) of the polygon mirror and a plane mirror (25) perpendicular to the rotation axis. A convergent radiation beam is projected on the convex mirror (24). The reflected radiation is subsequently focused on a detection system (40, 45) via an astigmatic imaging system (75, 78, 51). A substantially parallel radiation beam is incident on the plane mirror (25), which beam, after reflection, is also focused to a radiation spot (34a) on the detection system (40, 46). The position and the shape of the radiation spots (34, 34a) are a measure of the position and the orientation of the polygon mirror (10). After processing, the output signals of the radiation detection system are applied to elements (91), for example electromagnets for stabilizing the polygon mirror (10).

BACKGROUND OF THE INVENTION

The present invention relates to a device for optically determining theposition of an object. The device includes a radiation source unit forgenerating a radiation beam, a radiation-sensitive detection system andan optical imaging system arranged in the radiation path of theradiation beam for forming a radiation spot on the detection system. Theoptical imaging system is coupled entirely or partly to the object insuch a way that the position of the radiation spot on the detectionsystem is a measure of the position of the object in a directiontransverse to the radiation path.

A device of this type may be used, inter alia, for determining theposition of a freely suspended scanning mirror in a scanning device. Theinvention therefore also relates to apparatus for recording and/orreading information in an optical record carrier, which device comprisesa polygon mirror rotatable about an axis for scanning the recordcarrier.

In apparatus for optically recording or reading large quantities ofinformation on a record carrier by a scanning spot, this scanning spotshould cover a large distance per unit of time on the record carrier.This is necessary because the information density on the record carrieris bounded by the scanning spot dimension which in its turn isdetermined by the wavelength of the radiation and the numerical apertureof the scanning system. Both magnitudes cannot be chosen arbitrarily.For example, when a television program of HDTV quality is or has beenstored in a digitized form on the optical record carrier and if thelinear information density on the record carrier is approximately 1.5bit/μm, the scanning speed of the scanning spot should be approximately60 m/s. Even when using a limited number of parallel scanning spots, thescanning speed is more than 10 m/s. To reach such a speed using ascanning mirror, for example, a rotating polygon mirror, and to keep thescanning spot accurately aimed at the positions to be scanned, aconstant measurement of the polygon or mirror position and feedback tothe control mechanism of the mirror is necessary.

Such a measurement is to be preferably performed in a contactless way,for which, according to the invention, a device as described in theopening paragraph can be used.

Such a device is described per se from EP-A-0, 124, 145. Thisapplication describes an optical measuring system in which the end of afirst optical fiber is imaged by a lens or a concave mirror on a planein which the ends of the two other optical fibers are located. The firstoptical fiber is connected to a radiation source and the second otherfibers are connected to detectors. The imaging lens or mirror is coupledto a membrane whose displacement results in a displacement of the lensor mirror transversely to the direction of the incident radiation beam.Consequently, the radiation spot which has been formed is also displacedwith respect to the ends of the two optical fibers so that the relativeradiation intensity on the two detectors changes. In this way therelative intensity received by the detectors is a measure of theposition of the object in a direction transverse to the radiation pathto which the lens is coupled.

In the conventional device it is not possible to detect the displacementof the imaging system in a direction parallel to the radiation beam. Foran accurate determination of the position of an object, such as apolygon mirror or another scanning mirror, it is, however, necessary tomeasure the position in three dimensions in a rapid and reliable mannerwith a minimum possible number of additional optical systems. Possibleadditional optical elements should preferably not be coupledmechanically to the scanning mirror because this would lead to anincrease of the mass and the complexity of the system of the scanningmirror.

SUMMARY OF THE INVENTION

The invention has, inter alia, for its object to provide a devicegenerally described in the opening paragraph in which the location orposition of the object can also be determined in the direction of theradiation beam with the aid of only one optical imaging system. Suchdevice according to the invention is characterized in that the device isalso provided with means for changing the radiation spot on thedetection system in dependence upon the position of the object in thedirection of the radiation path.

The location or position of the object is determined by measuring thechange in shape of the radiation spot with the aid of theradiation-sensitive detection system. Simultaneously, the position ofthe object in the direction transverse to the radiation beam is detectedby the position of the radiation spot on the detection system.

A first illustrative embodiment of the device according to the inventionis characterized in that an astigmatic element for introducingastigmatism into the radiation beam is arranged in the radiation pathbetween the optical imaging system and the detection system. Thedetection system is adapted to detect the shape of the radiation spot.The position of the object can be determined with sufficient accuracy bymeans of astigmatism. It is to be noted that it is known per se fromU.S. Pat. No. 4,023,033 to correct the position of a lens in a radiationbeam in the radiation direction by an astimatic technique. However, inthe disclosed device the radiation beam is focused on a reflectingsurface by an objective lens. In this case the position of the lensitself is not important, but the relative distance between the lens andthe surface is important. Furthermore, the lens is fixed with respect tothe radiation source and the detection system in the directiontransverse to the direction of radiation.

This embodiment may be further characterized in that the optical systemis adapted to form a convergent beam at the location of the astigmaticelement and in that the astigmatic element is a plane-parallel platewhich is arranged obliquely in the radiation path. The embodiment mayalso be characterized in that the astigmatic element is a hologram or acylindrical lens. In both cases the astigmatic element may be connectedto the object or it may be fixed with respect to the radiation sourceunit and the detection system. With a view to mass reduction, the latteris preferred.

A second illustrative embodiment of the device according to theinvention is characterized in that a roof prism for forming tworadiation spots on the radiation-sensitive detection system is arrangedin the radiation path between the optical imaging system and thedetection system, the distance between the radiation spots being ameasure of the position of the object in the direction of the radiationpath. In this way the position of the imaging system in the direction ofthe radiation beam can also be measured. It is to be noted that it isknown per se, for example, from EP-A 0,063,830 or U.S. Pat. No.4,533,826 to correct the position of a lens relative to a reflectivesurface by a roof prism. However, also in this case the position of theobjective lens is not important but the correct focusing of a beam on areflective surface is important, and the lens cannot move in a directiontransverse to the direction of the radiation beam.

An illustrative embodiment of the detection system is furthercharacterized in that the optical imaging system has at least onereflective element. Consequently, the radiation source unit and thedetection system can be placed close to each other from a constructivepoint of view and it is not necessary to take an ongoing radiation beaminto account at the object. The imaging system may thus be provided at aside of the object facing the radiation source and the detection system.Moreover, space for the radiation path should only be available at oneside of the object.

The device according to the invention may be further characterized inthat the reflective element of the optical imaging system comprises aconcave or a convex mirror having a curved surface. The concave orconvex mirror forms the optical imaging system or is a part of it. Theuse of a convex mirror may have the constructive advantage that thereflective element can be arranged on a face of the object, for example,by means of glueing without the object itself having to be subjected toa separate treatment.

The device is preferably characterized in that the concave or convexmirror has a spherical shape. A rotation of the object around anarbitrary axis through the center of the sphere does then not have anyinfluence on the measurement of the position. Moreover, a sphere can beeasily made.

In a preferred illustrative embodiment of the device according to theinvention, the device is characterized in that the optical imagingsystem has a further portion which is arranged in a fixed positionrelative to the radiation source and the detection system. The imagingsystem thus comprises a portion which is not coupled to the object sothat less stringent requirements need to be imposed on the portion whichis coupled to the object. For example, the volume and mass of theportion coupled to the object can be minimized, while an optical systemof high quality comprising a plurality of elements can nevertheless beused.

In addition to the location or position of the object, the orientationor tilt of the object in the space is important, for example, fordetermining where the scanning beam reflected by a scanning mirror isincident on a surface to be scanned.

A device according to the invention which does not only determine theposition but also the tilt of the object is further characterized byalso having optical means for deflecting radiation from the radiationbeam in dependence upon the tilt of the object being coupled to theobject and by the detection system being further adapted to detect thedeflected radiation and hence the tilt of the object.

An illustrative embodiment is further characterized in that the meansfor deflecting radiation are reflective means. The means for measuringthe tilt are constituted, for example, by a plane mirror. This mirrorcan be simply combined with a reflective portion of the imaging system.

It is to be noted that it is disclosed per se from U.S. Pat. No.4,829,175 to determine the position of the rotation axis of a polygonmirror by means of a radiation beam which is reflected on an end face ofthe polygon. However, in this conventional device the position of thepolygon mirror in the space is not simultaneously measured so that onlylimited information about the polygon mirror is available. In thatdevice the mirror position is determined by a mechanical bearing. Amechanical bearing limits the maximum rotation speed and is undesirableat high rotation speeds.

The portions for determining the position and the orientation of theobject may be combined, for example, in an illustrative embodiment whichis characterized in that the optical imaging system has a furtherportion which is arranged in a fixed position relative to the radiationsource and the detection system, which further portion is integratedwith a beam splitter for splitting the radiation beam into two sub-beamsfor determining the position and the tilt, respectively, of the object.

In this manner the generated radiation beam is not split into a beam fordetermining the position and the tilt until the radiation is incident onthe imaging system. The radiation path between the radiation source andthe imaging system is singular. In a further embodiment the radiationpaths of the deflected radiation may also extend in common and thuscomprise common optical components.

One field of use or application of a device for determining the positionand/or the tilt of an object according to the invention is thedetermination of these magnitudes for a polygon mirror which isrotatable about an axis and is used in an apparatus for opticallyscanning an object or an area, for example, for scanning an opticalrecord carrier.

According to the invention, such apparatus for recording and/or readinginformation in an optical record carrier comprises a radiation sourceunit, a detection system and an imaging system which has at least onepart coupled to the polygon mirror for forming a radiation spot on thedetection system, the radiation source unit, the detection system andthe imaging system forming part of a device for determining the positionof the polygon mirror and/or the position of the rotation axis of therotatable polygon mirror. Such a polygon mirror is used to scan parallelstrips on the record carrier at a high speed so that it is necessary tomonitor the position of the polygon mirror and the position of the axiscarefully so as to avoid that strips which have already been scanned arescanned once more, or that strips to be scanned are skipped.

An illustrative embodiment of such apparatus comprises a rotatablepolygon mirror, with a rotationally symmetrical concave or convex mirrorbeing arranged around the rotation axis. By arranging the concave orconvex mirror, which is used to determine the position of the polygonmirror, rotationally symmetrically around the rotation axis, therotation of the polygon mirror about the rotation axis does not have anyinfluence on the measurement of the position.

According to the invention apparatus for recording and/or reading anoptical record carrier by means of a rotatable polygon mirror ispreferably implemented in a way in which the rotatable polygon mirror ismagnetically journalled by means of a plurality of electromagnets, whichapparatus is further adapted to apply energizing signals to theelectromagnets in dependence upon output signals of the detectionsystem.

In this case the polygon mirror is freely suspended in a magnetic fieldso that there is no friction with bearings when it rotates. The polygonmirror and the rotation axis are held in their correct positions withthe help of a position detection device as described in the foregoing.

These and other more detailed aspects of the invention will now bedescribed in greater detail with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing

FIG. 1 shows diagrammatically a first illustrative embodiment of adevice according to the invention;

FIG. 2 shows an alternative illustrative embodiment in which areflective optical element is used;

FIGS. 3a, 3b and 3c show diagrammatically some other illustrativeembodiments for the reflective optical element;

FIG. 4 provides illustrative embodiment in which also the tilt of theobject is measured; and

FIG. 5 shows an illustrative embodiment for stabilizing a rotatingpolygon mirror.

DETAILED DESCRIPTION OF THE PREFERRED

The reference numeral 10 in FIG. 1 denotes an object which can movefreely within certain boundaries. The object 10 is coupled to a lens 20whose position is directly related to the position of the object 10. Thelens 20 is secured, for example, to the object or connected to thisobject via a lever, a beam, or a shaft. A radiation source unit 30 isarranged at one side of the lens 20 and a detection system 40 isarranged at the other side. The radiation source unit 30, the detectionsystem 40 and the lens 20 are positioned with respect to one another insuch a way that the radiation source unit 30 is imaged on the detectionsystem 40 when the object and the lens are in their nominal positions.The radiation source unit comprises, for example, a lamp 31, a condensorlens 32 and a diaphragm 33.

The detection system 40 is a position-sensitive detection system andcomprises, for example, four radiation-sensitive diodes 41, 42, 43 and44 arranged in a square. The lens 20 is implemented in such a way thatit exhibits astigmatism so that the shape of the radiation spot 34projected on the detection system 40 depends on the position of the lens20 in the direction of the principal axis of the radiation beam comingfrom the radiation source unit 30 and being incident on the lens 20.

When the object 10 and hence the lens 20 are displaced in the directionof the double-headed arrow 11, the x or the y direction transverse tothe direction of the radiation beam, the spot 34 formed on the detectionsystem 40 is displaced in the same direction as the lens 20. When theobject 10 and hence the lens 20 are displaced in the direction of thearrow 12, the z direction, i.e. in a direction parallel to that of theradiation beam, the shape of the spot 32 changes from a circle to alying ellipse or a standing ellipse because the lens 20 is a lens has aconsiderable extent of astigmatism. The position of the lens 20 andhence that of the object 10 may be defined in three directions from theshape and the position of the radiation spot 34. Astigmatism may notonly be introduced into the radiation beam by means of the lens 20 butalso by means of an extra element 53, which is not connected to theobject 10 for example a grating, a hologram or a cylindrical lens.

To be able to derive three signals from the radiation spot 34; whichsignals define the position of the lens in the x, y and z directions,the detection system 40 is subdivided into four detectors 41, 42, 43 and44 which are arranged in a square and in which the bounding linesbetween the detectors extend at an angle of 45° to the axes of the lyingor standing elliptical shape of the radiation spot 34. The detectionelements 41 and 42 are juxtaposed in the y direction and the detectionelements 43 and 44 are juxtaposed in the x direction. The signalsdefining the position of the lens can then be derived from the outputsignals of the detection system via the relations:

    S.sub.x =(I.sub.43 -I.sub.44)

    S.sub.y =(I.sub.41 -I.sub.42)

    S.sub.z =(I.sub.41 +I.sub.42)-(I.sub.43 +I.sub.44)

in which S_(x), S_(y) and S_(z) are signals defining the displacementwith respect to a nominal position and I₄₁, I₄₂, I₄₃ and I₄₄ indicatethe intensity detected by each detection element. To define the exactposition from these signals S_(x), S_(y), and S_(z), a correction shouldbe carried out due to influences such as the intensity of the beam andthe effect of the pupil of the lens on the displacement and shape of theradiation spot.

FIG. 2 shows an illustrative embodiment in which the optical system isreflective. A concave mirror 21 is secured to the freely movable object10. A radiation source 30, shown in FIG. 2 as a semiconductor laser,generates a radiation beam which is incident on the concave mirror 21via a partially transparent mirror 50. The beam reflected thereby issubsequently incident on a roof prism 52 through the plane-parallelplate 51. The roof prism splits the radiation beam into two sub-beamseach of which forms a radiation spot 35, 36, respectively, on theradiation-sensitive detection system 60. The mutual distance between thetwo radiation spots 35 and 36 is a measure of the position of theconcave mirror 21 and the object 10 in the direction of the radiationbeam, the z direction.

The detection system 60 comprises, for example, eightradiation-sensitive elements 61, 62, 63, 64, 65, 66, 67 and 68constituting two parallel rows of four elements each and being arrangedin such a manner that the bounding line between the row comprising theelements 61, 63, 65 and 67, and the row comprising the elements 62, 64,66 and 68 coincides with the nominal positions of the radiation spots 35and 36. The pairs 61, 62 and 63, 64 are arranged in such a way that theyare located on both sides of the nominal position of the radiation spot35. The pairs 65, 66 and 67, 68 are arranged in an analogous manner withrespect to the radiation spot 36.

The position of the concave mirror 21 can be defined from the radiationdistribution on the detection elements in the x, y and z directions inaccordance with:

    S.sub.x =(I.sub.62 +I.sub.64 +I.sub.66 +I.sub.68)-(I.sub.61 +I.sub.63 +I.sub.65 +I.sub.67)

    S.sub.y =(I.sub.63 +I.sub.64 +I.sub.67 +I.sub.68)-(I.sub.61 +I.sub.62 +I.sub.65 +I.sub.66)

and

    S.sub.z =(I.sub.61 +I.sub.62 +I.sub.67 +I.sub.68)-(I.sub.63 +I.sub.64 +I.sub.65 +I.sub.66).

The transparent plane-parallel plate 51 on which the partiallytransparent mirror 50 is arranged is located in a convergent radiationbeam in the embodiment shown. As a result, astigmatism is introducedinto the radiation beam after the plane-parallel plate 51, so that theposition of the concave mirror 21 in the z direction can also bedetermined by the astigmatic method as illustrated with reference toFIG. 1.

FIGS. 3a, 3b and 3c show diagrammatically some other embodiments of amirror connected to the object for use in accordance with the invention.In FIG. 3a the radiation beam generated by the radiation source 30 isformed to a parallel beam by means of a lens 71, which beam is incidenton the concave paraboloid mirror 22 via the partially transparent mirror50. This paraboloid mirror focuses the beam on the detection system 40where a radiation spot 34 is formed. In FIGS. 3b and 3c the radiationbeam is focused in the focal point near the object 10 by means of thelens 71. A spherical mirror 23 or 24, concave 23 in FIG. 3b and convex24 in FIG. 3c, is connected to the object 10 and the central point ofthe spherical shape of this mirror coincides with said focal point whenthe mirror 23 or 24 is in its nominal position.

The radiation beam is reflected by the mirror 23 or 24 and after it hastraversed the beam splitter or partially transparent mirror 50 it isfocused by the lens 73 to a radiation spot 34 on the detection system40. Astigmatism is introduced into this beam, for example, by arrangingan oblique plane-parallel plate 51 in a convergent or divergent part ofthe beam. A cylindrical lens or a grating 53 may also be arranged in thebeam. The distortion of the radiation spot 34 which is caused thereby issubsequently detected on the detection system 40, for example, in themanner as described with reference to FIG. 1.

Apart from a paraboloid or spherical shape, the mirror may also have ahyperboloid or ellipsoid shape. In that case the radiation beam isfocused on the first focus of the hyperboloid or ellipsoid by means ofthe lens 71 and the second focus is coincides with the detection systemor is imaged thereon via a further optical imaging system.

In the embodiments shown the position of the radiation spots on thedetection system is only determined by the position of the object andnot by its orientation.

FIG. 4 shows a first embodiment of a system with which the orientationof the object can also be determined. In this embodiment the position isdetermined by way of example with the aid of a convex mirror 24, as inFIG. 3c. A plane mirror 25 has been arranged in addition to the convexmirror 24. The device is provided with a condensor lens 71 with whichthe radiation beam generated by the radiation source is converted into aparallel beam. This parallel beam is deflected in the direction of theobject 10 via reflection on the beam splitter 50. An objective lens 72with which a part of the beam is focused to a point coinciding with thenominal position of the centre of the convex mirror 24 is arranged inthis beam. As is shown in the Figure, this can be achieved in that thelens 72 only covers a part of the cross-section of the radiation beam.It is, for example, alternatively possible to arrange an optical wedgein the radiation beam with which a part of the beam is guided past theobjective lens 72. The part of the radiation beam traversing the lens 72and being reflected on the curved mirror 24 is projected on thedetection system 40 via the beam splitter 50 and the lens 73 forsupplying a signal from which the position of the object 10 can bederived. The other part of the beam is incident on the plane mirror 25and, after it has traversed the beam splitter 50 and the lens 73, itforms a second radiation spot 34a on the detection system. The positionof this radiation spot 34a almost exclusively depends on the tilt of theplane mirror 25 and does not depend or hardly depends on its position.By providing a wedge or prism in one of the two radiation beam partswhich are reflected by the mirrors 24 and 25, respectively, theradiation spots 34 and 34a are spatially separated from each other sothat they are detected independently of each other on two portions 45and 46 of the detection system 40. In the illustrative embodiment ofFIG. 4 this is realized by arranging an optical element 74 in theradiation beam, which element is wedge-shaped in the portion which istraversed by radiation from the plane mirror 25 and which has a constantthickness in the rest of the beam.

In FIG. 4 beam splitter 50 may not only be implemented as a splittingcube but also as a partially transparent mirror and be supported by aplane-parallel plate by means of which astigmatism is introduced intothe beam between the object 10 and the detection system 40. A preferredillustrative embodiment is depicted in FIG. 5 wherein an application ofthe position detection device is shown for stabilizing a rotatingpolygon mirror. FIG. 5 includes a semiconductor laser 30 for generatingthe radiation beam and a partially transparent mirror 50 for deflectingthe radiation beam towards the object 10. A collimator lens 78 withwhich the beam is made parallel and a lens 75 for focusing the radiationbeam on the curved mirror 24 on the object 10 are arranged between thepartially transparent mirror 50 and the object 10 in this embodiment.The lens 75 has a plane central portion 76, which plane and acorresponding plane 77 in the other reflective surface of the lensextend at a small angle to each other so that this portion functions asan optical wedge. The lens 75 focuses the light incident on theperipheral refractive surface towards a point which coincides with thecentral point, or a focal point, of the convex mirror 24 on the object10. This radiation is reflected by the mirror 24 and focused on thedetection system 40 via the lenses 75 and 78 and the plane-parallelplate 51. In the manner described hereinbefore a radiation spot 34,whose position and shape provide information about the position of themirror 24 and hence of the object 10, is formed on the portion 45 of thedetection system 40.

The radiation which is incident on the plane central portion 76 of thelens 75 is not focused on the convex mirror 24 but is incident on thereflective surface 25 around it. This radiation is reflected thereon,while the direction of the reflected radiation exclusively depends onthe tilt of the plane mirror 25 and hence of the object 10. Thereflected beam again traverses the wedge which comprises the faces 76and 77 in the lens 75, subsequently it traverses the lens 78 and thepartially transparent mirror 50 and forms a radiation spot 34a on theportion 46 of the radiation-sensitive detection system 40. The positionof this spot 34a defines the tilt of the plane mirror 25 and hence thetilt of the object.

The radiation-sensitive detection system 40 comprises, for example, twoquadrant detectors 45 and 46 each consisting of four radiation-sensitiveelements with which the position as well as the shape of the radiationspots 34 and 34a formed is determined.

An application of the position and orientation detector according to theinvention is also shown in FIG. 5. The object 10 is, for example, apolygon mirror which is rotatable about a shaft 11. The polygon mirrorhas a plurality of facets, shown in the Figure as faces 12 extending atan angle of 45° to the rotation shaft 11. A radiation beam 80 from aradiation source 81 and the condensor lens 82 is incident on thereflective faces 12 of the polygon mirror and is deflected thereby,dependent on the position of the polygon. The radiation beam is focusedto a radiation spot 85 on a surface 84 to be scanned via a lens system83, for example, an f-θ lens. This surface forms part of, for example,an optical record carrier which is recorded or read by means of thescanning beam 80. The record carrier is, for example, a disc-shaped ortape-shaped record carrier which is recorded by means of a large numberof parallel, relatively short tracks or strips. The direction of thetracks or strips is determined by the combined displacement of therecord carrier 84 with respect to the scanning device and the movementof the scanning spot 85 due to the rotation of the polygon.

To be able to record or read information on the record carriersufficiently rapidly in this manner, for example, for a HDTV program(high-definition television), the polygon should rotate at a speed ofseveral thousand revolutions per second. To achieve this, the polygon ismagnetically journalled and secured to a metal or magnetized disc 90which is held in position and driven by a plurality of electromagneticcoils 91.

The radiation spots 34 and 34a which are formed on the detection system40 via the reflective surface 25 and the convex mirror 24 are convertedby the detection elements in the system 40 into electric signals whichprovide information about the position and tilt of the polygon and areanalysed in a processing unit 92. This processing unit subsequentlysupplies output signals 93 which are applied to the electromagnets 91with which the magnetic fields generated thereby are influenced. Theposition and the tilt of the rotating polygon mirror is kept constantthereby.

To increase the scanning speed or to realise a lower rate of revolutionof the polygon, it is possible to implement the optical system 81-83 insuch a way that the record carrier simultaneously scans a plurality ofparallel tracks by means of a plurality of scanning spots.

We claim:
 1. A device for optically determining the position andorientation of an object, comprising a radiation source unit forgenerating a radiation beam along a radiation path, aradiation-sensitive detection system, and an optical imaging systemarranged in the radiation path of said beam for forming a radiation spoton the detection system, said optical imaging system being coupled atleast partly to said object in such a way that the position of theradiation spot on the detection system is a measure of the position ofthe object in a direction transverse to the radiation path,characterizedin that the device further comprises a curved-surface optical elementfixed to said object, and an astigmatic element for introducingastigmatism into the radiation beam in the radiation path between saidoptical imaging system and said detection system, and said detectionsystem comprises means for detecting the shape of the radiation spot,said shape being a measure of the position of the object in thedirection of the radiation path.
 2. A device as claimed in claim 1,characterized in that the optical imaging system (21; 22; 23; 24) has atleast one reflective element.
 3. A device as claimed in claim 1,characterized in that the optical imaging system has at least a furtherlens (72; 75) arranged in a fixed position relative to the radiationsource (30) and the detection system (40; 60).
 4. A device as claimed inclaim 1, wherein optical means (25) for deflecting radiation from theradiation beam in dependence upon the tilt of the object (10) is coupledto the object, and the detection system (40; 60) is further adapted todetect the deflected radiation and hence the tilt of the object.
 5. Adevice as claimed in claim 1, characterized in that said curved-surfaceoptical element is a curved-surface mirror fixed to said object.
 6. Adevice for optically determining the position and orientation of anobject, comprising a radiation source unit for generating a radiationbeam along a radiation path, a radiation-sensitive detection system, andan optical imaging system arranged in the radiation path of said beamfor forming a radiation spot on the detection system, said opticalimaging system being coupled at least partly to said object in such away that the position of the radiation spot on the detection system is ameasure of the position of the object in a direction transverse to theradiation path,characterized in that the device further comprises acurved-surface optical element fixed to said object, and roof prismarranged in the radiation path between said optical imaging system andsaid detection system, for forming two radiation spots on said radiationdetection system, and said detection system comprises means fordetecting the distance between the radiation spots, said distance beinga measure of the position of the object in the direction of theradiation path.
 7. A device as claimed in claim 6, characterized in thatsaid curved-surface optical element is a curved-surface mirror fixed tosaid object.
 8. A device for optically determining the position andorientation of an object, comprising a radiation source unit forgenerating a radiation beam along a radiation path in a given direction,a radiation-sensitive detection system, and an optical imaging systemarranged in the radiation path of said beam for forming a radiation spoton the detection system, said optical imaging system being coupled atleast partly to said object in such a way that the position of theradiation spot on the detection system is a measure of the position ofthe object in a direction transverse to the radiation path,characterizedin that the device further comprises means for changing the shape of theradiation spot on the detection system as a function of the position ofthe object in said given direction, said means including acurved-surface optical element fixed to said object.
 9. A device asclaimed in claim 8, characterized in that the optical imaging system hasat least a further lens (72; 75) being arranged in a fixed positionrelative to the radiation source (30) and the detection system (40; 60).10. A device as claimed in claim 8, wherein optical means (25) fordeflecting radiation from the radiation beam in dependence upon the tiltof the object (10) is coupled to the object and the detection system(40; 60) is further adapted to detect the deflected radiation and hencethe tilt of the object.
 11. A device as claimed in claim 10,characterized in that the means for deflecting radiation are reflectivemeans (25).
 12. A device as claimed in claim 10, characterized in thatthe optical imaging system has a further portion (75) which is arrangedin a fixed position relative to the radiation source (10) and thedetection system (40), which further portion is integrated with a beamsplitter (76, 77) for splitting the radiation beam into two sub-beamsfor determining the position and the tilt, respectively, of the object.13. A device as claimed in claim 11, characterized in that the opticalimaging system has a further portion (75) which is arranged in a fixedposition relative to the radiation source (10) and the detection system(40), which further portion is integrated with a beam splitter (76, 77)for splitting the radiation beam into two sub-beams for determining theposition and the tilt, respectively, of the object.
 14. A device asclaimed in claim 8, characterized in that the optical imaging system(21; 22; 23; 24) has at least one reflective element.
 15. A device asclaimed in claim 14, characterized in that the optical imaging systemhas at least a further lens (72; 75) arranged in a fixed positionrelative to the radiation source (30) and the detection system (40; 60).16. A device as claimed in claim 14, characterized in that thereflective element of the optical imaging system comprises a concave ora convex mirror (21; 22; 23; 24).
 17. A device as claimed in claim 16,characterized in that the optical imaging system has at least a furtherlens (72; 75) arranged in a fixed position relative to the radiationsource (30) and the detection system (40; 60).
 18. A device as claimedin claim 16, characterized in that the concave or convex mirror (23; 24)has a spherical shape.
 19. A device as claimed in claim 18,characterized in that the optical imaging system has at least a furtherlens (72; 75) arranged in a fixed position relative to the radiationsource (30) and the detection system (40; 60).
 20. A device as claimedin claim 8, characterized in that said curved-surface optical element isa curved-surface mirror fixed to said object.
 21. A device as claimed inclaim 20, characterized in that the optical imaging system (21; 22; 23;24) has at least one reflective element.
 22. A device as claimed inclaim 20, characterized in that the optical imaging system has at leasta further lens (72; 75) arranged in a fixed position relative to theradiation source (30) and the detection system (40; 60).
 23. A device asclaimed in claim 20, wherein optical means (25) for deflecting radiationfrom the radiation beam in dependence upon the tilt of the object (10)is coupled to the object, and the detection system (40; 60) is furtheradapted to detect the deflected radiation and hence the tilt of theobject.
 24. A device as claimed in claim 20, characterized in that theoptical system (20) is adapted to form a convergent beam at the locationof the astigmatic element and in that the astigmatic element is aplane-parallel plate (51) which is arranged obliquely in the radiationpath.
 25. A device as claimed in claim 24, characterized in that theoptical imaging system (21; 22; 23; 24) has at least one reflectiveelement.
 26. A device as claimed in claim 24, characterized in that theoptical imaging system has at least a further lens (72; 75) arranged ina fixed position relative to the radiation source (30) and the detectionsystem (40; 60).
 27. A device as claimed in claim 24, wherein opticalmeans (25) for deflecting radiation from the radiation beam independence upon the tilt of the object (10) is coupled to the object,and the detection system (40; 60) is further adapted to detect thedeflected radiation and hence the tilt of the object.
 28. A device asclaimed in claim 20, characterized in that the astigmatic element (53)is a hologram or a cylindrical lens.
 29. A device as claimed in claim28, characterized in that the optical imaging system (21; 22; 23; 24)has at least one reflective element.
 30. A device as claimed in claim28, characterized in that the optical imaging system has at least afurther lens (72; 75) arranged in a fixed position relative to theradiation source (30) and the detection system (40; 60).
 31. A device asclaimed in claim 28, wherein optical means (25) for deflecting radiationfrom the radiation beam in dependence upon the tilt of the object (10)is coupled to the object, and the detection system (40; 60) is furtheradapted to detect the deflected radiation and hence the tilt of theobject.